Conceptual comparison: nuclear fission is most directly the opposite process to which of the following?

Introduction / Context:
Nuclear energy can be released either by splitting heavy nuclei (fission) or by joining light nuclei (fusion). These processes occupy opposite ends of the binding-energy-per-nucleon curve: very heavy and very light nuclei can move toward the iron peak, releasing energy when transitioning to more tightly bound configurations.

Given Data / Assumptions:

• Fission: a heavy nucleus (e.g., U-235) splits into lighter fragments plus neutrons.
• Fusion: light nuclei (e.g., deuterium and tritium) combine to form heavier nuclei.
• Energy release governed by changes in nuclear binding energy.

Concept / Approach:
On the mass–energy landscape, fission moves from heavy to intermediate masses; fusion moves from light to intermediate masses. Both release energy when products have higher binding energy per nucleon than the reactants. Therefore, the “opposite” of fission is fusion, not generic radioactive decay or non-nuclear phenomena.

Step-by-Step Solution:
Define fission and fusion in terms of nuclear mass numbers.Relate energy release to binding-energy differences.Identify the direct inverse transformation: heavy-to-lighter versus light-to-heavier.Select nuclear fusion as the opposite process.

Verification / Alternative check:
Binding-energy curves show a peak around iron–nickel; moving toward this peak from either side (heavy via fission, light via fusion) releases energy, reinforcing the conceptual opposition of the two processes.

Why Other Options Are Wrong:

• Radioactive decay: Includes alpha, beta, gamma processes; not simply the inverse of fission.
• Thermionic emission: Electron emission from heated metals; not nuclear.
• Combustion: Chemical oxidation; unrelated to nuclear binding energies.
• Photoelectric emission: Quantum electron ejection; not nuclear.

Common Pitfalls:
Conflating “radioactivity” broadly with all nuclear processes; forgetting that decay can also occur in fission fragments but is not the inverse of fission itself.

Final Answer:
Nuclear fusion